Sources of portable stored electrical power may be in the form of electrochemical devices. One such electrochemical power source is the battery. A battery may comprise one or more electrochemical cells which are capable of converting stored chemical energy into usable electrical energy. In a conventional battery, there are two electrodes, one being the cathode, the other being the anode, each of which may be connected to a current collector. Depending upon whether the battery is consuming or providing electrical power, the cathode and anode may be either a positive terminal or a negative terminal. For example, when a battery is discharging (providing) power, the cathode is provides the positive terminal of the battery, while the anode provides the negative terminal. Conversely, when a battery is recharging (consuming) power, the cathode provides the negative terminal of the battery, while the anode is the positive terminal of the battery. The electrolyte is positioned between anode and cathode allowing ions move between terminals enabling flow of electric current.
Batteries also often include an electrolyte which is sufficiently ionized to conduct an electric current between the respective electrodes. The electrolyte may be presented in a liquid, solid, or semi-solid (gel) form, and may also require a separator/membrane within the electrolyte to prevent the battery from short-circuiting. Batteries may also comprise half-cells in which different electrolytes are present. As such, batteries may also be classified as being either primary batteries (i.e., are non-rechargeable or “disposable”) or secondary batteries (i.e., may be recharged and reused).
Another example of an electrochemical device which may be used to provide electrical power is an electrochemical capacitor, often referred to as a “supercapacitor.” Similar to batteries, supercapacitors comprise two electrodes in contact with an electrolyte between these electrodes. This electrolyte may also be a solid, liquid, gel, etc. In supercapacitors, these two electrodes may also be separated by an ion permeable separator (e.g., membrane). At the interface of each electrode with the electrolyte is formed a double layer, known as Helmholz double layer. In supercapacitors, the primary electrical current storage occurs at each of these Helmholz double layer interfaces. One of the advantages of supercapacitors (relative to conventional batteries) is the ability of such supercapacitors to store and deliver energy at rates far beyond those attainable by conventional batteries. In addition, the longer life cycle of supercapacitors, as well as the higher charge/discharge rate at higher power densities enable the use such supercapacitors in power back-up systems, consumer portable devices, electrical/hybrid automobile, construction machinery, etc.